If Microsoft decides to give us access to the APIs to implement a window manager (however you'd do that on Windows...), possible. As is, would require a significant redesign of our approach.
That being said, tethered mode will be Windows compatible.
By and large, having access to Windows APIs is rarely a problem. Since Vista they have locked down the OS a bit so you may have to ask for more permissions (or straight admin). More importantly is: are all APIs you need documented? You can still use undocumented calls but they won't be stable.
Replacement 'window managers' were a bit of a craze years ago. I remember running a version of OpenSTEP.
That being said, tethered mode will be Windows compatible.